The treatment of liquid hydrocarbons containing undesirable acidic species such as mercaptans is known and can be performed using either an extraction or a conversion process. The conversion processes are known as “sweetening” processes where an aqueous solution containing a mixture of an alkali metal hydroxide, such as sodium hydroxide, and a chelated metal catalyst is contacted with a hydrocarbon stream in the presence of an oxygen containing gas. An oxidation reaction occurs that converts the mercaptans to disulfide oils, which remain in the hydrocarbon phase during a subsequent step to separate the hydrocarbon from the aqueous solution. These sweetening processes work effectively on light hydrocarbon feeds with light mercaptan impurities.
The extraction processes, such as described in U.S. Pat. Nos. 6,860,999; 6,960,291; 7,014,751; and 7,029,573, requires liquid-liquid mass transfer of the mercaptans from the hydrocarbon to an aqueous solution under anaerobic conditions, i.e., in the substantial absence of added oxygen. Such processes were especially effectively for removal of high molecular weight mercaptans (C4 and higher) that are typically contained in heavier liquid hydrocarbon feeds. The aqueous solution preferably has two phases where alkylphenols, such as cresols (in the form of the alkali metal salt), are combined with a polyvalent metal catalyst, and an alkali metal hydroxide in an aqueous extractant phase and a denser aqueous bottom phase that is substantially immiscible in the extractant. The alkylphenols were used to enhance the extraction of the heavier mercaptans. The metal catalyst is included in the solution to minimize entrainment of the aqueous solution in the treated hydrocarbon, particularly at the higher viscosities encountered at higher alkali metal hydroxide concentration. During mixing with a “sour” liquid hydrocarbon feed, the mercaptans are physically extracted (not converted) into the aqueous extractant phase, and after separation an upgraded hydrocarbon product is obtained that is substantially lower than the feed in mercaptan content. The extractant phase aqueous solution is then sent to an oxidation process where an oxygen containing gas is added and the metal catalyst present in the solution converts the mercaptans to disulfides. These alkylphenol based extraction processes are more complicated and difficult to operate principally because of the need to use a two-phase aqueous extraction solution, or a single phase compositionally located at the phase boundary between the one and two-phase regions.
There remains a need, therefore, for new hydrocarbon treatment processes that minimize operational difficulty and minimize the need for secondary processes to treat sulfur contaminants.